Åke Johansson: The Proterozoic evolution of Blekinge and Bornholm

During the last few years, improved methods for U-Pb age determinations, and especially the possibility to date individual growth stages in single zircon crystals using the NORDSIM ion microprobe, have led to dramatically increased knowledge about the geological evolution of the Southwest Swedish Gneiss Province / Sveconorwegian Province (e.g. Söderlund 1999). The importance of the c. 1.45 Ga Hallandian thermomagmatic event (term originally coined by Hubbard 1975) has been underlined, both in terms of metamorphic overprinting with new zircon growth in older rocks, and intrusive activity with crystallization of new granitoids. However, the tectonic significance of this event is still poorly understood, in terms of whether this represents an orogenic event accompanied by deformation, or whether it is an essentially anorogenic thermomagmatic event. One reason for this is the later Sveconorwegian (c. 0.95 Ga) overprinting in Southwest Sweden, as well as the possiblity that older deformation is Gothian (c. 1.55-1.70 Ga). This has led to a heated debate on the evolution of Southwest Sweden between those who want to emphasize the importance of the Gothian episode, not just as a crust-forming event, but to form the main structure of Southwest Sweden by collision and accretion of magmatic arcs (e.g. Åhäll 1995, 1996, Åhäll et al. 1995, Christoffel et al. 1999), and those stressing the importance of Sveconorwegian high-grade metamorphism and deformation (e.g. L. Johansson et al. 1996, Andersson et al. 1999, Söderlund 1999 and references therein).

One way of resolving these problems is to cross the Protogine Zone / Sveconorwegian Frontal Deformation Zone, and go eastwards into an area that has escaped the Sveconorwegian overprinting, i.e. to Blekinge. Here, the relative importance of Gothian versus Hallandian tectono-magmatic activity can be easier evaluated. In an earlier NFR-supported project by me, the broad geochronological framework for the early evolution of the Blekinge province was outlined, with 1.7-1.8 Ga magmatic rocks (Västanå volcanites, ‘coastal gneiss´, Tving granite; Johansson & Larsen 1989) being deformed prior to the intrusion of the c. 1.45 Ga old Karlshamn-Spinkamåla granites (Åberg 1988 and references therein). Additional geochronological information from Blekinge has been published by Kornfält (1993, 1996). The younger, ‘anorogenic´ granites in southeast Sweden have been investigated by Åberg (1988 and references therein), and are the focus of a research project led by Svetlana Bogdanova (Lund University), who correlates them with unexposed magmatic rocks within the Baltic Republics and NE Poland (Mazury Complex). These ‘anorogenic´ granites have similar ages as the c. 1.45 Ga Hallandian event in southwest Sweden, and may thus be an upper crustal manifestation of the same thermomagmatic activity.

The Danish island of Bornholm is located in the Baltic Sea south of Blekinge, and has a Precambrian bedrock composed of gneisses and granitoids which is probably a continuation of the bedrock of the Blekinge province. At the same time, Bornholm forms an important stepping stone, connecting the bedrock of southern Sweden with the concealed Precambrian basement of northeast Poland and Lithuania, investigated through geophysics and in drill cores within the international EUROBRIDGE project. However, very little modern geochronological information exists from Bornholm. From the older gneisses, which tentatively may be correlated with the 1.7-1.8 Ga gneisses of Blekinge, no radiometric age data exist at all. From the younger, intrusive granites, a composite Rb-Sr isochron has yielded an age of 1400 ± 60 Ma (Larsen 1980), while K-Ar ages range between 1255 ± 15 Ma and 1340 ± 30 Ma (Larsen 1971), probably reflectng partial resetting of the K-Ar system after crystallization. Some of these granites were sampled by Stefan Claesson at LIG a few years ago, and conventional U-Pb zircon ages exist for two of them: 1469 ± 26 Ma (MSWD 18) for the Rønne granite, considered to be the oldest of the intrusive granites on geological grounds, and 1452 ± 19 Ma (MSWD 19) for the Svaneke granite, considered to be the youngest granite (S. Claesson, unpub. data). However, the high MSWD values indicate considerable scatter, and these analyses will need refinement. Also, more granite samples need to be analysed, and the older gneisses need to be sampled and dated.

The aim of the present project is to focus on the older (Gothian or TIB-equivalents) metamorphosed magmatic rocks in Blekinge, using earlier samples already dated by conventional multigrain U-Pb (Johansson & Larsen 1989) complemented by new samples with good structural control, and apply refined dating methods, especially the NORDSIM ion microprobe, to obtain better determined magmatic crystallization ages and to study the effect of metamorphic overprinting. Of special concern is the age of metamorphism. U-Pb dating of titanite may be especially useful for determining metamorphic ages. On Bornholm, both the older gneisses and the younger granites need to be dated, using conventional U-Pb or NORDSIM spot analyses of zircon and titanite. These will be complemented by major and trace element geochemistry and Sm-Nd isotope analyses, in order to determine the petrogenesis and tectonic setting of these rocks, and get some insight into the time of original crustal formation in Bornholm. Through cooperation with Svetlana Bogdanova and Audrius Cecys from Lund University, and Hemin Koyi from Uppsala University, structural data may also be integrated in the interpretation of the results.

Correlation of events across the Protogine Zone is of major importance, since the areas on either side of this tectonic zone probably represent different exposed crustal levels through the same bedrock province. Correlation between the southeast Fennoscandian Shield (southern Sweden) and the concealed Precambrian basement of the East European Platform (NE Poland, Lithuania etc.) has been the focus of the EUROBRIDGE project (Bogdanova et al. 1996). Large-scale E-W-trending zones, one of which separates Blekinge from the more undeformed TIB granitoids to the north, continue in the basement of the Baltic States and Russia, and may play an important role in the tectonic framework of the region and for controlling c. 1.45 Ga magmatism in the whole area (cf. Bogdanova & Cecys 1999).

On a wider scale, in terms of large-scale geotectonic reconstructions, the project is related to IGCP-project 440 (Assembly and break-up of the Rodinia supercontinent), and more specifically to the pre-assembly stage of the Neoproterozoic Rodinia supercontinent (cf. Åhäll & Connelly 1998, Weil et al. 1998, Karlstrom et al. 1999). The project will thus aid in defining the position of Fennoscandia within the presumed Mesoproterozoic supercontinent, its role during the break-up of this supercontinent and its reassembly into the Neoproterozoic Rodinia supercontinent during the Grenvillian-Sveconorwegian orogenic cycle.

Results (Nov 2009)

Results on U-Pb dating of the Bornholm granitoids and gneisses, indicating similar crystallization ages of 1450 to 1470 Ma for all rock units, has been published in International Journal of Earth Sciences (Zarins & Johansson, 2009: U-Pb geochronology of gneisses and granitoids from the Danish island of Bornholm: new evidence for 1.47-1.45 Ga magmatism at the southwestern margin of the East European Craton. International Journal of Earth Sciences (Geologische Rundschau), vol. 98, p. 1561-1580, DOI 10.1007/s00531-008-0333-0). A manuscript on the geochemistry of the Bornholm rocks was also included in Kristina Zarins licentiate thesis (Department of Earth Sciences, Uppsala University, 2007).

Contact:
Åke Johansson